Patent Literature 1 discloses a probe which is to be attached to the fingertip of a subject. The probe includes a light-emitting element and a light-detecting element. The light-detecting element has a light-detecting surface configured to detect light that has been emitted from the light-emitting element, and that has been transmitted through the living tissue of the fingertip. The light-detecting element is configured to output a signal corresponding to the intensity of the light which has been detected by the light-detecting surface. The light emitted from the light-emitting element is determined to have a wavelength which is to be absorbed by a material in blood. The volume of blood in the fingertip is changed in accordance with the pulse, and therefore the intensity of the light which is detected by the light-detecting surface is changed as well. The signal output from the light-detecting element is used for calculating biological information of the subject, such as the pulse and the arterial oxygen saturation.